The present invention relates generally to prosthetic implants for reconstructing the shoulder joint. More specifically, the invention relates to a glenoid component for a shoulder prosthesis and a method for affixing the glenoid component to a scapula in an implant procedure.
The glenoid cavity is located on the upper external border of the scapula between the acromion process and the coracoid process on a boney formation known as the scapula head. The glenoid cavity is a shallow, pear shaped, articular surface whose longest diameter is in the proximal-distal direction. It is broader distally than proximally, and its apex is a slight impression, the supra-glenoid tubercle, to which is attached the long tendon of the biceps muscle. The cavity is covered with cartilage and its margins, slightly raised, give attachment to a fibro-cartilaginous structure, the glenoid ligament, by which its cavity is deepened. The glenoid cavity articulates with a large, rounded head at the proximal end of the humerus. The head is nearly hemispherical in form and is directed proximally and medially and slightly posteriorly. Its surface is smooth and coated with cartilage.
Multiple forces are applied to the glenoid cavity and are accounted for in designing a glenoid prosthesis. Typically, the glenoid resurfacing component is made of ultrahigh molecular weight polyethylene (UHMWPE) having a concave laterally facing articulating surface and a convex medial bone contacting surface, which may include multiple pegs. The pegs resist the various types of loading placed on the glenoid by the head portion of the humeral component allowing the glenoid component to offer a stable and secure articulating surface. Typically, the pegs are inserted into a hole bored in the glenoid cavity and are secured either by bone cement or by being press fit in the bores.
Typically, the medial surface of the glenoid component includes three to five pegs to stabilize and secure the glenoid component to the scapula. The glenoid components may offer one of two larger diameter pegs and multiple pegs especially on the distal medial surface of the glenoid component. The pegs on the glenoid component are located and oriented for placement within the scapula at locations where maximum amounts of natural bone are available for effective anchoring of the pegs and for minimal risk of deleterious bone perforation, and for managing moments exerted on the glenoid component as a result of the forces encountered during service. For more effective resistance to separation of the glenoid component from the scapula, the glenoid component typically employs a full complement of the fixation pegs combined with an overall curved affixation surface for resisting shear forces and rocking the glenoid on the scapula, while preserving existing natural bone at the implant site.
Typical glenoid components are shown in U.S. Pat. Nos. 4,964,865; 5,573,448; 6,379,386; and 6,911,047.